Giant-chirp oscillator

ABSTRACT

A laser apparatus operable to generate giant chirp pulses. The pulses have a centre frequency comprising an arrangement of components connected or connectable to form a closed ring cavity. The components comprise a first gain medium, an optical isolator, a length of single mode fibre, a mode locking device, an output coupler, and an optical filter. Each of the components are optical fibre based components.

FIELD OF THE INVENTION

The invention relates to a fibre laser and in particular to a giantchirp oscillator.

BACKGROUND TO THE INVENTION

A problem with fibre lasers in the prior art is they are often toounstable for use in industrial applications.

OBJECT OF THE INVENTION

It is an object of the present invention to provide a fibre laser whichgoes at least some way toward overcoming the abovementioned disadvantageor which at least provides the public with a useful choice.

Other objects of the invention may become apparent from the followingdescription which is given by way of example only.

SUMMARY OF THE INVENTION

In a first aspect the present invention relates to a laser apparatusoperable to generate an optical pulse comprising a sequentiallyconnected arrangement of optical fibre based components to form a closedring cavity including at least one gain medium, an optical isolator, alength of single mode fibre, a mode locking device, an output couplerand an optical filter.

In another aspect the present invention relates to a passively modelocked, ring fibre laser comprising a loop of polarisation maintaining,all normal dispersion single mode optical fibre, the loop including asection of polarisation maintaining rare-earth doped fibre gain medium,a length of polarisation maintaining single mode fibre and apolarisation maintaining nonlinear optical loop mirror.

In another aspect the present invention relates to a passively modelocked laser comprising two rings of polarisation maintaining all normaldispersion optical fibre, the first ring comprising a polarisationmaintaining normal dispersion first gain medium, a polarisationmaintaining normal dispersion optical filter, a polarisation maintainingnormal dispersion optical isolator, and a length of polarisationmaintaining normal dispersion single mode fibre, the second ringcomprising a polarisation maintaining normal dispersion second gainmedium, and wherein the first ring is optically coupled to the secondring by a wavelength division multiplexing optical coupler.

In another aspect the present invention relates to a laser apparatusoperable to generate giant chirp pulses having a centre frequencycomprising an arrangement of components connected, or connectable toform a closed ring cavity, the components comprising a first gainmedium, an optical isolator, a length of single mode fibre, a modelocking device, an output coupler, and an optical filter, wherein eachof the components are optical fibre based components.

In another aspect the present invention relates to a laser operable as agiant chirp oscillator, comprising a loop of substantially polarisationmaintaining, substantially normal dispersion, optical fibre basedcomponents including a gain medium, a section of single mode fibre, andan nonlinear amplifying optical loop mirror.

In another aspect the present invention relates to a laser operable as agiant chirp oscillator, comprising at least two loops of polarisationmaintaining, all normal dispersion optical fibre, a first loopcomprising a polarisation maintaining normal dispersion first gainmedium, a polarisation maintaining normal dispersion optical filter, apolarisation maintaining normal dispersion optical isolator, and alength of polarisation maintaining normal dispersion single mode fibre,a second loop comprising a polarisation maintaining normal dispersionsecond gain medium, and wherein the first loop is optically coupled tothe second loop by a wavelength division multiplexing optical coupler.

In another aspect the present invention relates to a laser as claimed inclaim 30 wherein the first loop further comprises wavelength divisionmultiplexing optical coupler to couple a source of pump light into thefirst gain medium, the second loop further comprises wavelength divisionmultiplexing optical coupler to couple a source of pump light into thesecond gain medium, and at least one of the sources of pump light areoperable to mode lock the oscillator.

Preferably the optical fibre components are polarisation maintainingcomponents.

Preferably the components have, or substantially have, a normaldispersion relative to the centre frequency.

Preferably the components have a net normal dispersion relative to thecentre frequency.

Preferably the gain medium is operable to provide energy to apropagating pulse when pumped with a pump light source.

Preferably the mode locking device is passive.

Preferably the mode locking device is a nonlinear amplified loop mirrorcomprising at least an optical coupler operable to provide and input andan output, and a second gain medium operable to provide energy to apropagating pulse.

Preferably the second gain medium is operable to provide energy to apropagating pulse when pumped with light from a second pump lightsource.

Preferably the mode locking device is operable to mode lock the laser byvarying the energy of the light from the second pump source.

Preferably the length of single mode fibre is operable in the cavity toreceive light from the first gain medium and output light to the secondgain medium.

Preferably the length of single mode fibre is operable to impartbroadening to a pulse propagating within it.

Preferably the laser is operable to generate giant chirp pulsescomprising a chirp that is substantially linear and positive.

Preferably varying the length of the single mode fibre varies at leastone or more of the pulse repetition rate, the pulse energy or the pulseduration.

Preferably the optical filter has an operating bandwidth centred at orapproximately at a desired pulse wavelength.

Preferably the output coupler couples up to 80% of the pulse energy fromthe cavity.

Preferably the length of single mode fibre is at least 0.5 m.

Preferably the length of single mode fibre is up to 1000 m.

Preferably the pulses are compressible to less than 120 fs.

Preferably the pulses have a duration of at least 4 ps.

Preferably the pulses have a duration of up to 150 ps.

Preferably the pulses have a repetition rate of at least 1.4 MHz.

Preferably the pulses have a repetition rate of up to 15 MHz.

Preferably the pulses have spectral bandwidth of at least 3 nm.

Preferably the pulses have spectral bandwidth of up to 26 nm.

Preferably the pulses have spectral bandwidth of up to 40 nm.

Preferably the pulses have energy of at least 10 nJ.

Preferably the pulses have energy of up to 20 nJ.

Preferably the length of single mode fibre has a mode area up to 20 μm.

In another aspect the present invention relates to a method of operatinga laser comprising providing the laser of any one of the abovestatements, providing pump light to each gain medium in the laser, andadjusting the power of the pump light to achieve mode locking.

Other aspects of the invention may become apparent from the followingdescription which is given by way of example only and with reference tothe accompanying drawings.

As used herein the term “and/or” means “and” or “or”, or both.

As used herein “(s)” following a noun means the plural and/or singularforms of the noun.

The term “comprising” as used in this specification and claims means“consisting at least in part of”. When interpreting statements in thisspecification and claims which include that term, the features, prefacedby that term in each statement, all need to be present but otherfeatures can also be present. Related terms such as “comprise” and“comprised” are to be interpreted in the same manner.

It is intended that reference to a range of numbers disclosed herein(for example, 1 to 10) also incorporates reference to all rationalnumbers within that range (for example, 1, 1.1, 2, 3, 3.9, 4, 5, 6, 6.5,7, 8, 9 and 10) and also any range of rational numbers within that ramie(for example, 2 to 8, 1.5 to 5.5 and 3.1 to 4.7).

The term “single mode fibre” as used within this specification means afibre capable of supporting a single mode of propagation and maycomprise fibres comprising a ‘larger than is currently standard’ modearea of, for example, 20 μm squared or more.

The entire disclosures of all applications, patents and publications,cited above and below, if any, are hereby incorporated by reference.

To those skilled in the art to which the invention relates, many changesin construction and widely differing embodiments and applications of theinvention will suggest themselves without departing from the scope ofthe invention as defined in the appended claims. The disclosures and thedescriptions herein are purely illustrative and are not intended to bein any sense limiting.

In this specification, where reference has been made to external sourcesof information, including patent specifications and other documents,this is generally for the purpose of providing a context for discussingthe features of the present invention. Unless stated otherwise,reference to such sources of information is not to be construed, in anyjurisdiction, as an admission that such sources of information are priorart or form part of the common general knowledge in the art.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be described by way of example only and withreference to the drawings in which:

FIG. 1 shows one preferred embodiment of the laser adapted to form aring or cavity laser known as a Giant Chirp Oscillator (GCO).

FIG. 2 shows another preferred embodiment of the laser.

FIG. 3(a) shows a graph of fibre length verses RMS pulse width and pulseenergy.

FIG. 3(b) shows a graph of fibre length verses wavelength and spectralintensity.

FIG. 4(a) shows a simulated output pulse profile versus SMF length.

FIG. 4(b) shows a graph of pulse power and chirp for a 150 m length offibre.

FIG. 4(c) shows a graph of pulse power and chirp for a 70 m length offibre.

FIG. 5(a) shows a plot of the temporal evolution of a pulse propagatingthough the laser.

FIG. 5(b) shows a plot of the spectral evolution of the pulsepropagating though the laser.

FIGS. 6(a) and 6(b) show the laser output spectrum.

FIGS. 6(c) and 6(d) show the temporal profiles retrieved from a FROGtrace before a grating pair.

FIGS. 6(e) and 6(f) show the temporal profiles retrieved from a FROGtrace after a grating pair.

FIG. 7 shows another example of a laser configuration.

FIG. 8 shows the measured autocorrelation of the base laserconfiguration showing a measured pulse width of 120 fs.

FIG. 9 shows the normalised spectrum and actual spectrum (inset) of thelaser configuration of FIG. 7 as measured with an optical spectrumanalyser.

DETAILED DESCRIPTION OF THE INVENTION

A laser constructed from optical components each having normaldispersion is known as an all-normal dispersion (ANDi) laser and aparticular operating regime of an ANDi laser is known as a giant chirposcillator (GCO). A GCO has a laser cavity with a large optical lengthsuch that a pulse propagating within the laser has a large net normaldispersion effected upon it. A GCO typically produces a pulse that istemporally broad, has a low-repetition rate, high-energy and a verylarge linear chirp compared to other lasers.

Pulses from a GCO are useful for a wide range of applications such asmicromachining, nonlinear imaging and spectroscopy. The possibility tocontrol the pulse characteristic over a wide range is particularlyimportant for chirped pulse amplification systems where, for instance,the possibility to design a seed oscillator with low repetition rate mayeliminate the need for other external elements such as a pulse picker.

However, pulsed GCO lasers known in the prior art have been mode lockedusing either nonlinear polarization evolution (NPE) or saturableabsorber (SA) mechanisms. Both NPE and SA mechanisms have a number ofsignificant disadvantages that make them unsuitable for use as part of alaser in an industrial application. One disadvantage of an NPE is thatit requires ongoing polarization angle adjustment. An NPE is generallyrestricted for use in a non-polarisation maintaining laser cavity. Onedisadvantage of a SA is that it suffers from limited longevity andlimited power handling.

Only particular base lasers configurations can be converted into GCOlasers. Whether a base laser is a suitable candidate for becoming a GCOlaser requires consideration of the interplay of effects betweendispersion, nonlinearity and gain.

The invention allows the disadvantages of using an SA to be at leastpartially overcome by using a nonlinear amplifying loop mirror (NALM) asthe mode-locking element.

Efforts to construct ANDi lasers mode-locked with a NALM have not beensuccessful. One effort has produced a figure-eight laser that requiresan external input to initiate pulsed operation. An external input can beprovided by an electrically driven modulator to initiate mode lockingand therefore pulsed operation. However, external modulation inputs addunwanted complexity and cost to a laser.

A further disadvantage associated with ANDi lasers mode-locked with aNALM is the low pulse energy, <2 nJ, and high repetition-rate, >33 MHz,thus making the laser inappropriate for many applications.

A further disadvantage of the figure-eight ANDi lasers mode-locked witha NALM is that the repetition rate cannot simply be modified without thelaser becoming unstable. The pulse energy cannot be increased byamplification due to excessive breaking of the pulse that preventsmode-locking.

A further disadvantage associated with ANDi lasers mode-locked with aNALM is that the pulses produced by the laser has a pulse energy limited<2 nJ. Further amplification of those pulses results in significant wavebreaking. The prior art lasers are also only able to couple a smallportion of light out of the cavity which restricts the power the lasercan generate. The remaining portion of light must remain within thecavity to maintain lasing.

A further disadvantage associated with ANDi lasers mode-locked with aNALM is that the laser must have short overall fibre lengths therebydictating relatively high repetition rate and pulses with a short outputpulse duration. High repetition rates and short output pulse durationcauses significant peak power increase when passing through anamplifier. The large peak power of the pulse causes significant amountsof detrimental wave breaking.

Lasers constructed entirely from fibre based optical components havedesirable qualities including compactness, lack of sensitivity toalignment and low production cost. However, mode lockable fibre basedlasers that contain free-space and/or non-polarization maintainingelements are usually undesirable for use in industrial environments asthey are critically hampered by their sensitivity to thermal andmechanical induced stress. Stress is known to change the birefringencecharacteristics of a non-polarization maintaining fibre based laser andcause degradation in laser performance or outright failure of the laser.Lasers constructed using non fibre based optical components, or freespace components, or non-all-polarization maintaining elements requirehighly sophisticated packaging and construction to be suitable forindustrial use due to their extreme sensitivity to environmentalconditions and frequent tuning requirements.

Preferred embodiments of the invention relate to a GCO laser constructedentirely from polarization maintaining fibre based optical components. Abenefit of using all fibre components is that the laser is impervious tothermally and/or mechanically induced stress. One example of the laseris operable to deliver pulse energies up to approximately 20 nJ withoutcompromising the compressibility of the pulses below 500 fs. The lasermay be tuned to adjust the output pulse energy, pulse duration andrepetition rates to suit various applications. The laser may alsooperate at one of a variety of wavelengths by a selection of appropriateoperating frequencies of components in the cavity. The laser may alsooffer the further advantage of generating output pulses that can readilybe compressed to produce high peak power and short duration.

A GCO regime can be achieved when a length of single mode fibre (SMF) isinserted into the cavity. A detrimental build-up of excessive peak powerin the cavity can be avoided by propagating the pulse through the lengthof SMF. The length of SMF allows the cavity to sustain pulses withlarger energies that do not undergo wave-breaking by allowing the pulseto temporally broaden and maintain or reduce in amplitude. The length ofSMF therefore allows for a significant increase in output pulse energywhen further amplified under particular conditions.

The length of the various segments of SMF between components of thelaser is of particular importance. The significance of the segments ofSMF between components necessitates consideration of even trivial items,such as patch cords, which may otherwise be overlooked. Any variation inthe lengths of SMF between components can lead to trivial or non-trivialmodifications of the pulse propagation and thus affect thecharacteristics of the output pulse.

An advantage of the laser is that it comprises two gain mediums. Asecond gain medium implemented in the NALM is used to enablemode-locking and also provides further amplification of the pulse inaddition to a first gain medium. The laser output characteristics may bevaried by varying the relative gain provided by the two gain mediums.

Modulators to drive a mode locking device and devices to drivemodulators increases the cost of manufacture and can increase thephysical size of the laser. A further common issue with activemode-locking is that the driving signal must be quite precisely matchedwith the cavity length (longitudinal mode spacing). As the cavity lengthcan change from thermal perturbations, a feedback loop is required tokeep the modulator frequency matched which requires some form ofadditional electronics.

The laser according to preferred embodiments is passive in that it willadvantageously mode lock without an external modulation source. Removingthe need for a modulator reduces manufacturing cost and allows a smallerform factor. In one example the laser advantageously provides arelatively high pulse energy (>10 nJ) and low-repetition rate (<5 MHz)making it ideal as a front-end in CPA systems.

The laser is operable as a giant chirp oscillator, and has a loop ofsubstantially polarisation maintaining, substantially normal dispersion,optical fibre based components including a gain medium, a section ofsingle mode fibre, and an nonlinear amplifying optical loop mirror. Pumplight is provided to each of the gain mediums to initiate mode lockingand sustain pulsed operation.

The laser may also have at least two loops of polarisation maintaining,all normal dispersion optical fibre. A first loop has a polarisationmaintaining normal dispersion first gain medium, an polarisationmaintaining normal dispersion optical filter, a polarisation maintainingnormal dispersion optical isolator, and a length of polarisationmaintaining normal dispersion single mode fibre. A second loop has apolarisation maintaining normal dispersion second gain medium. Thesecond loop is a NALM. The first loop is optically coupled to the secondloop by a wavelength division multiplexing optical coupler.

The first loop further has wavelength division multiplexing opticalcoupler to couple a source of pump light into the first gain medium. Thesecond loop has a wavelength division multiplexing optical coupler tocouple a source of pump light into the second gain medium. At least oneof the sources of pump light is operable in that the pump power can bevaried to mode lock the oscillator. Pump light to the gain mediumsshould be maintained to sustain mode locked operation.

FIG. 1 shows one embodiment of the laser adapted to form a ring orcavity laser 1 known as a Giant Chirp Oscillator. The laser 1 is asequential arrangement of optically coupled optical fibre basedcomponents. The laser 1 comprises a fibre based gain medium 10, a fibrebased isolator 20, a length of single mode optical fibre 30, a fibrebased mode locking device 40, a fibre based output coupler 50 adapted toprovide an output 60 from the cavity, and a fibre based band-pass filter70. The output of the filter 70 is connected to the input of the gainmedium 10 to close the cavity. The particular order of the components inthe cavity has some flexibility as will be discussed below.

Gain Medium

The gain medium 10 receives energy from a pump source (not shown). Thegain medium 10 is operable to transfer energy from the pump to anoptical pulse that is circulating in the cavity. Propagation of anoptical pulse in the cavity is sustained when the gain medium 10provides more energy to a circulating pulse than is lost by the pulseduring its round-trip in the cavity.

The gain medium 10 can be provided by a number of different devices suchas a single mode fibre, Ytterbium (Yb), an Erbium (Er), Neodymium,Holmium other rare-earth doped fibres. Those skilled in the art willappreciate the particular gain medium used will be related to thedesired output wavelength of the pulses to be generated and sustained bythe cavity. Further, those skilled in the art will appreciate theparticular pump source required to enable a particular fibre to act as again medium will depend on the particular gain medium selected.

Preferably a rare earth-doped fibre is used as a gain medium as theyusually provide greater amplification compared to a single-mode fibres(using Raman or parametric gain).

For example, if the desired output wavelength is around 1 micrometre,the potential rare-earth doped fibres gain medium that are operable toprovide amplification at this wavelength are Ytterbium, or Neodymiumdoped fibres as they provide amplification of light around 1 micrometre.

In another example, if the desired output wavelength is around 1.5micrometres, an Erbium-doped fibre would provide amplification aroundthis wavelength provided that suitable normal dispersion fibres at thatwavelength were used. Similarly, if the desired output wavelength isaround 2 micrometres, a Thulium or Holmium-doped fibre would provideamplification.

In another example, if the desired output wavelength is around 1micrometer, the gain medium selected can be a 5 meter long Yb-dopedfibre pumped by a light source such as a laser diode of approximately976 nm. A Yb-doped fibre gain medium is also preferable as they providea broader emission band (advantageous for ‘ultra short’ pulses), highoptical conversion efficiency, do not saturate easily and can be pumpedwith common telecommunications diodes.

Isolator

The isolator 20 is arranged in the cavity 1 such that light willpropagate only in one direction. Preferably the output of the isolator20 is coupled to the input of the length of SMF 30. However, therelative position of the isolator 20 and length of SMF 30 can be swappedas desired.

Length of SMF

The overall length of fibre in the cavity determines the pulse energy,pulse duration and repetition rate of the laser. The laser is preferablyconstructed entirely from polarization maintaining, single mode fibreand polarization maintaining optical components. It has been determinedthat polarization maintaining fibres provide the significant advantageof stabilising the cavity against environmental influences such asthermal and mechanical stress. Different types of SMF fibres may be usedas long as the fibres substantially maintain a constant polarizationstate and have normal dispersion.

The SMF 30 preferably has a length of at least 0.5 m and may extend tohundreds of metres, and may extend to 1000 m. The output pulse energy,output pulse duration and the repetition rate of the pulses from thelaser can be tuned by changing the length of the SMF 30.

The output of the length of SMF 30 is preferably coupled to the input ofthe mode locking device 40. However, it may be arranged elsewhere in thecavity with appropriate tuning changes made to the remaining devicesthat complete the cavity.

Mode Locking Device

The mode locking device 40 is adapted to enable the laser to operate ina pulsing condition rather than a continuous wave light source. The modelocking device 40 may be either an active device or a passive device.

An active device operates to promote pulsed operation by modulating theloss in a length of fibre or by modulating the phase of lightpropagating in the fibre. However, an active mode locking devicerequires an external driving source. A passive mode locking device ispreferable as it does not require an external input to function andgenerally uses internally propagating light to create modulation.Passive mode locking devices are generally cheaper and easier to operatethan active mode locking devices.

The mode-locking device 40 introduces losses that decrease as the powerof the light that propagates within it increases. In this way, lowoptical powers experience a large loss and optical high powersexperience small loss. The selectivity of the mode locking elementpromotes production of short optical pulses having a high power (and lowlosses) over a continuous optical wave having a low power (largelosses).

A mode locking device may be implemented by several devices ortechniques, such as nonlinear-polarization evolution (NPE), a saturableabsorber (SA) a nonlinear optical loop mirror (NOLM) or a nonlinearamplified loop mirror (NALM).

It has been determined that saturable absorbers have severaldisadvantages including being prone to damage and once manufacturedtheir operational parameters cannot be modified. Saturable absorbersmust therefore always be chosen so as to suit the characteristics of aparticular cavity. If the laser cavity does not mode lock, or does notmode-lock with the desired pulse characteristics, another saturableabsorber must be purchased or the cavity modified. A NOLM suffers thesame disadvantages.

A disadvantage of NPE is that it requires frequent adjustment ofpolarisation to maintain the pulse characteristics or to re-initiatemode-locked laser operation. Lasers utilising NPE suffer from long termreliability problems caused by thermal and mechanical perturbations.Small disturbances induce random fluctuations in fibre birefringencecausing failure of mode-locking or changes in the output pulsecharacteristics in a non-polarization maintaining cavity. Lasers usingNPE are not suitable for industrial applications due to the presence ofsources of perturbations.

The inventors have determined the issue of polarisation stability can beovercome with the use of a NALM. A NALM does not suffer frompolarisation maintaining limitations and allows construction ofpolarisation maintaining cavity whilst providing a degree of freedomthrough which the mode-locking ability can be tuned. A NALM, inpreferred configurations of the laser, provides mode-locking and allowsa robust polarisation maintaining cavity to be produced. The cavity isadvantageously substantially insensitive to environmental influencessuch as thermal changes and vibration.

The preferred mode locking device 40 is a NALM. The preferred form ofthe NALM comprises an optical coupler, a gain medium and a length ofSMF. One example of a NALM comprises a 55/45 coupler, a 0.5 m longsegment of highly doped Yb fibre and a 2 m length of SMF. Preferably theYb-doped fibre is operable to be pumped with a laser diode at 976 nm.Preferably the output of the mode locking device 40 is operativelycoupled to the input of the output coupler 50.

The gain medium 10 and the NALM are separated by the SMF 30 that has alength significant enough to broaden the temporal pulse width therebyreducing the pulse peak power such that the pulse shape is substantiallyretained when further amplified in the NALM. The SMF 30 allows for thepulses to extract more energy from the pump in each gain medium withoutundergoing detrimental wave-breaking.

Output Coupler

The output coupler 50 provides a point in the cavity from which laseroutput power can be extracted and diverted to an output 60. Onepreferred form of the output coupler 50 is approximately an 80/20coupler which diverts approximately 80% of the power to output 60 whilemaintaining approximately 20% inside the cavity. A wide range of outputcoupler ratios could be implemented. For example, a 70/30 coupler couldbe used.

Filter

Typically in a fibre laser the cavity boundary condition is satisfied byexciting a solution such that the pulse is unchanged at the beginning ofeach cavity round-trip. Alternatively, the cavity boundary condition issatisfied by managing pulse dispersion with segments of anomalous andnormal dispersion such that the pulse broadens in the normal dispersionfibre segments and is compressed in the anomalous dispersion fibre. Eachof these techniques requires fibre with anomalous dispersion.

In an all-normal dispersion cavity the pulse is continuously broadenedthroughout the cavity and simultaneously acquires a positive chirp. Topreserve the cavity boundary conditions the pulse characteristics mustbe “reset” at the end of the round trip. The band pass filter 70 ensuresa pulse propagating in the cavity is substantially the same at thebeginning of each round trip. The filter 70 is operable to output anarrow spectral band of the pulse. Due to the all-normal cavity thepulses gather a strong positive chirp and consequently the spectralfiltering also results in temporal shortening of the pulse.

The filter has, for example, a 1.7 nm bandwidth centred at or near theoperation frequency. A filter with smaller or a larger bandwidth mayalso be used as desired. The filter may also be any component with aband-pass spectral transmission.

Operation

The length of SMF 30 acts as an intracavity pulse stretching mechanism.A pulse circulating in the cavity is amplified in the gain medium 10,temporally broadened by the length of SMF 30, and amplified andtemporally shaped by the NALM. This configuration allows for asignificant increase in pulse energy.

Tuning of the particular length of the SMF 30 advantageously allows thepulse characteristics to also be tuned. The energy and temporal durationof the output pulses will typically increase when the length of SMF isincreased. In particular, the length of SMF 30 is tuneable to temporallystretch the pulse between the two active fibres in the gain medium 10and the NALM. This advantageously allows for the gain medium in the NALMto amplify the pulse without inducing detrimental wave-breaking effects.Further, higher output powers can be achieved than with merely thesingle gain medium 10 in the cavity.

Experimental Data

FIG. 2 shows an example of a laser 2. The laser 2 has a gain medium 10constructed from a length of low doped Yb doped fibre 13. The fibre 13is pumped from a 976 nm diode based light source coupled into the gainmedium through a wide band coupler 11. The output of the gain medium 10is operatively coupled to the input of the length of SMF 31.

The length of SMF 30 is at least 0.5 m and may be up to 100 in lengthwithout affecting the ability of the laser to mode lock in thisconfiguration. The output of the length of SMF 31 fibre is connected toan optical isolator 21 which has an output connected to a mode lockingdevice 41.

In this example, the mode locking device is a NALM comprising a 55/45optical coupler 41 comprising a first connection operable to receivelight from the isolator 21, a second connection operable to couple lightto a length of SMF 44, a third connection operable to couple light to a0.5 m length of highly doped Yb doped fibre 45 and a fourth connectionoperable to output light from the NALM.

The ends of SMF 44 and doped fibre 45 are connected to a wide bandcoupler 43 that allows the Yb doped fibre 45 to be pumped by a 976 nmdiode based light source 42. The SMF 44 preferably has a length of atleast 2 m and may be up to 5 m. The output from the NALM is connected toan output coupler 51.

The output coupler 51 is operable to provide a first light output 61where 80% of the light is coupled from the cavity and a second outputthat is connected to a band pass filter 71. The filter 71 has abandwidth of 1.7 nm and preferably has a centre frequency locatedsubstantially at the desired pulse centre frequency. The output of thefilter 71 is connected to the input of the gain medium 10.

FIGS. 3 and 4 show an example of a simulated cavity output for thecavity shown in FIG. 2. In particular, FIG. 3(a) shows a graph of pulseenergy and r.m.s. duration of the simulated output pulses as a functionof the length of SMF (L_(SMF)) 31. The output energy initially increasesthen begins to saturate after approximately 30 m before abruptlytransitioning into a regime of linear growth.

FIG. 3(b) shows the output spectrum of the simulated output spectrumwhere the transition is also apparent, suggesting that at this SMFlength the oscillator transitions from one mode-locking regime toanother. The desired GCO-regime achieved for L_(SMF)>30 m corresponds tooutput pulses characterized by high energy, long temporal duration and alarge linear chirp.

FIG. 4 shows pulse temporal characteristics of the output pulse changefor increasing L_(SMF). In particular, FIG. 4(a) shows that as the SMFlength increases the output pulses become ever broader, yet retain analmost constant temporal shape. This is also confirmed in FIGS. 4(b) and4(c) where we plot the temporal profile in more detail for selectedL_(SMF). FIGS. 4(b) and 4(c) also show the frequency chirp associatedwith the output profiles and observe linear variation for both cases.The output pulse energy can be increased simply by increasing the lengthof the SMF 31. The pulse energy can exceed 10 nJ using short lengths ofSMF 31.

FIGS. 5(a) and 5(b) show the temporal and spectral evolution of thepulse inside the simulated cavity as shown in FIG. 2, over one roundtrip and for a fixed length of SMF 31 of 50 m. Note that only thecounter-clockwise propagation inside the NALM is shown for simplicity.FIG. 5(a) shows the amplified pulse undergoes significant stretching inthe section of SMF 31 before being re-amplified in the NALM. The SMF 31acts to temporally stretch the pulse between the two active fibres inthe gain medium 13 and the NALM 45, allowing for the gain medium in theNALM 45 to amplify the pulse without inducing detrimental wave-breakingeffects. FIG. 5(b) shows the spectral width of 1.7 nm at the beginningof the roundtrip is broadened during propagation to nearly is nm when itis incident to the filter 71. The majority of the spectral broadeningoccurs in the gain fibre 13, whilst evolution in the SMF isquasi-linear.

FIG. 6 shows the experimentally recorded laser output spectrum (a, b)and temporal profiles retrieved from a FROG trace before (c, d) andafter (e, f) grating pair compression for both the 50 m and 100 m SMFlengths. The experimental cavity substantially corresponds to theexample of the laser 2 shown in FIG. 2. The cavity is constructed usinga polarisation maintaining optical fibre splicer and the laser outputdiagnosed with a fast photodiode, optical and radio-frequency (RF)spectrum analyzers and a commercial frequency-resolved optical gating(FROG). A grating pair with 1200 lines/mm may be used to recompress theoutput pulses external to the cavity.

In order to further experimentally investigate the role of the SMF 31separating the main gain fibre 13 and the NALM 40, experiments wereperformed with a 50 m and 100 m length of PM-980 optical fibre.Advantageously, the laser 2 could be mode locked simply by adjustingeither or both of the pump diode drive currents 12, 42. The laser emitsa stable train of pulses when mode locked. The laser is operable toadvantageously provide approximately 60 dB signal-to-noise ratio of inthe first RF harmonic. The laser is insensitive to thermal andmechanical stress, and further, the laser can be operated withoutinterruptions for extended periods of time.

FIG. 7 shows another example of a laser configuration. The laser has again medium constructed from a 5 m long low doped Yb doped fibre. The Ybdoped fibre is preferably pumped from a 976 nm diode based light sourcecoupled into the gain medium through a wide band coupler. The output ofthe gain medium is coupled to the input of a length of SMF fibre. Theoutput of the SMF fibre is connected to an optical isolator. Theisolator is connected to a mode locking device. In this example, themode locking device is a NALM comprising a 55/45 optical coupler havinga first connection to receive light from the isolator, a secondconnection to couple light to a 5 m length of SMF, a third connection tocouple light to a 0.5 m length of highly doped Yb doped fibre, and afourth connection to output light from the NALM. The ends of SMF anddoped fibre are connected to a wide band coupler that allows the Ybdoped fibre to be pumped by a 976 nm diode based light source. Theoutput from the NALM is connected to an output coupler which provides afirst light output where 70% of the light is coupled out from the cavityand the second output 30% is connected to a band pass filter. The filterhas a bandwidth of 1.7 nm and preferably has a centre frequency locatedsubstantially at the desired pulse centre frequency. The output of thefilter is connected to the input of the gain medium.

FIG. 8 shows the measured autocorrelation of the base laserconfiguration showing a measured pulse width of 120 fs. A theoreticalGaussian profile (dashed line) is overlaid with the pulse to show aclose match.

FIG. 9 shows the normalised spectrum and actual spectrum (inset) of thelaser configuration of FIG. 7 as measured at the output with an opticalspectrum analyser.

The laser advantageously provides for tuning of the generated pulsecharacteristics simply by changing the length of the SMF section.Shortening the SMF section will produce larger repetition rates, lowerpulse energies and durations whereas lengthening the SMF will allow forthe delivery of larger pulse energies and durations but lesser averagepowers due to decrease in repetition rate.

For example, a length of SMF up to 100 m will create pulses having apulse energy, duration and repetition rate ranging between 2-15 nJ,10-70 ps and 10-1.5 MHz respectively. All SMF lengths allow for thepulses to be externally compressed to below 500 fs.

The preferred embodiments further provide a laser with a repetition ratethat can easily be set to a desired frequency, ‘f_(rep)’. The length Lof the SMF section can be approximated by L=c/(n*f_(rep)′) where ‘n’ isthe refractive index of silica and ‘c’ the speed of light. The lengthalso relates to the accessible pulse energy. Alternatively, if ‘X nJ’pulse energy is desired then the SMF length can be approximated by 2.5nJ+0.15 nJ*L=X nJ, where X nJ is the target energy desired. Thoseskilled in the art will appreciate the ‘X nJ’ target will often requiresome experimental iteration to get the most desirable length.

The laser may also advantageously be tuned to a variety of operatingwavelengths. For example, the gain mediums may be implemented usingThulium or Holmium doped fibres to implement a ‘mid-infrared’ laser asthese fibres have a gain bandwidth operational for wavelengths greaterthan 1.9 μm. Those skilled in the art will appreciate fibres offeringnormal dispersion at the desired operational wavelength will be requiredfor the laser to operate. For example, a small core PCF silica fibre maybe used for an operating wavelength of approximately 1.9 μm.Alternatively, non-silica glass fibres may be used.

Industrial applications for the preferred embodiments of the laserinclude atmospheric transmission either for civil or militaryapplications where a transmission wavelength of around 2 μm wavelengthmay be desired as it is safe for eyesight. Other applications includespectroscopy, LIBS (Laser-Induced Breakdown Spectroscopy), medicalapplications such as eye surgery (Cornea surgery), material processingand especially plastic material which are transparent in the visible orat fpm, generation of an intense THz source, high order harmonicsgeneration and attosecond pulse generation.

To date energy range of pulses produced by the laser is in the range of0.2 nJ to 20 nJ. However, those skilled in the art will recognise thathigher pulse energies can be expected. For example, the use of ‘largecore’ (larger core than standard SMF) optical fibres could be used inthe cavity 1 to further increase pulse energy. Such large core fibresshould also be designed such that support a single mode. Such large corefibres typically have a core size of, for example, at least 10 μm. Somelarge core fibres have a mode area greater than 20 μm.

The oscillator has been shown to be operable to generate a pulse that istemporally broad, has a low-repetition rate, high-energy and a verylarge linear chirp. In addition, the pulses have a chirp that issubstantially linear and positive.

Other parameters of the output pulse include that: The pulses arecompressible to less than 120 fs. The pulses have duration of at least 4ps. The pulses have duration of up to 150 ps. The pulses have arepetition rate of at least 1.4 MHz. The pulses have a repetition rateof up to 15 MHz. The pulses have spectral bandwidth of at least 3 nm.The pulses have spectral bandwidth of up to 26 nm. The pulses havespectral bandwidth of up to 40 nm.

Wherein the foregoing description reference has been made to elements orintegers having known equivalents, then such equivalents are included asif they were individually set forth. Although the invention has beendescribed by way of example and with reference to particularembodiments, it is to be understood that modifications and/orimprovements may be made without departing from the scope or spirit ofthe invention.

1-32. (canceled)
 33. A laser apparatus operable to generate giant chirppulses having a center frequency, comprising an arrangement ofcomponents connected, or connectable to form a cavity, the componentscomprising a first gain medium, an optical isolator, a length of singlemode fibre, a passive mode locking device, an output coupler, and anoptical filter, wherein each of the components are optical fibre basedcomponents.
 34. A laser as claimed in claim 33, wherein the opticalfibre components are polarization maintaining components.
 35. A laser asclaimed in claim 33, wherein the components have, or substantially have,a normal dispersion relative to the center frequency.
 36. A laser asclaimed in claim 33, wherein the components have a net normal dispersionrelative to the center frequency.
 37. A laser as claimed in claim 33,wherein the mode locking device is a nonlinear amplified loop mirrorcomprising at least an optical coupler operable to provide and input andan output, and a second gain medium operable to provide energy to apropagating pulse.
 38. A laser as claimed in claim 33, wherein the modelocking device is operable to mode lock the laser by varying the energyof the light from the second pump source.
 39. A laser as claimed inclaim 33, wherein the length of single mode fibre is operable in thecavity to receive light from the first gain medium and output light tothe second gain medium.
 40. A laser as claimed in claim 33, wherein thelength of single mode fibre is operable to impart broadening to a pulsepropagating within it.
 41. A laser as claimed in claim 33, wherein thelaser is operable to generate giant chirp pulses comprising a chirp thatis substantially linear and positive.
 42. A laser as claimed in claim33, wherein varying the length of the single mode fibre varies at leastone or more of the pulse repetition rate, the pulse energy or the pulseduration.
 43. A laser as claimed in claim 33, wherein the optical filterhas an operating bandwidth centered at or approximately at a desiredpulse wavelength.
 44. A laser operable as a giant chirp oscillator,comprising a loop of substantially polarization maintaining,substantially normal dispersion, optical fibre based componentsincluding a gain medium, a section of single mode fibre, and annonlinear amplifying optical loop mirror.
 45. A laser operable as agiant chirp oscillator, comprising: a first lasing section comprisingall normal dispersion optical components and a second lasing sectioncomprising a nonlinear amplifying loop mirror, wherein the first andsecond lasing sections are operable to generate giant chirp pulses. 46.A laser as claimed in claim 45 wherein the first and second lasingsections each comprise a loop of polarization maintaining, all normaldispersion optical fibre, wherein a first loop comprises a polarizationmaintaining normal dispersion first gain medium, a polarizationmaintaining normal dispersion optical filter, a polarization maintainingnormal dispersion optical isolator, and a length of polarizationmaintaining normal dispersion single mode fibre, and a second loopcomprises a polarization maintaining normal dispersion second gainmedium, and the first loop is optically coupled to the second loop by awavelength division multiplexing optical coupler.
 47. A laser as claimedin claim 46 wherein the first loop further comprises wavelength divisionmultiplexing optical coupler to couple a source of pump light into thefirst gain medium, the second loop further comprises wavelength divisionmultiplexing optical coupler to couple a source of pump light into thesecond gain medium, and at least one of the sources of pump light areoperable to mode lock the oscillator.